1. Field of the Invention
The present invention relates to a hydrostatic continuously variable transmission in which a clutch valve capable of operating between a clutch-on position for setting a hydraulic closed circuit to a hydraulic transmitting state and a clutch-off position for setting the hydraulic closed circuit to a hydraulic transmission locking state is connected to the hydraulic closed circuit between a hydraulic pump and a hydraulic motor.
2. Description of the Prior Art
Conventionally, as such a continuously variable transmission, there has been known, for example, such a type of transmission as is disclosed in Japanese Patent Publication No. 3208/1966.
In such a continuously variable transmission, when the degree of a hydraulic transmission from a hydraulic pump to a hydraulic motor is controlled, the discharge amount of hydraulic oil from the high pressure portion of a hydraulic closed circuit is regulated by regulating the opening of a clutch valve by a clutch controller. However, if the viscosity of a working oil is varied by a temperature change, the discharge amount of the hydraulic working oil from the clutch valve is altered even if the clutch valve opening is not varied. Thus, there is a problem that the transmission ratio of the hydraulic transmission is varied.